Much effort has been devoted to the development of transmissive backlit displays for the flat panel display market. However reflective displays, which do not require backlighting, are desirable for low power applications and for paper-like displays that can be used as alternatives to the printed page. Displays with high reflectivity, high contrast, and colour capability are particularly desirable for such applications.
One approach has used electrophoretic display devices (EPDs), but there are problems with colour switching in conventional electrophoretic and other light-scattering systems. In some cases in these systems standard transmissive colour filters are used. Lower transmittance of the filters noticeably reduces brightness.
Most prior art electrophoretic devices are switchable between two colours but are not full colour devices. Examples are the E-ink electrophoretic display described in U.S. Pat. No. 6,262,707, or retroreflective electrophoretic displays described in EP 1 264 210. WO 2002/100155 describes an attempt to realise an electrophoretic device containing different pigments for multicolour switching. However the control of migration of different coloured pigments under different applied voltages is problematic and limits the practical fabrication of this device.
T. Endo, T. Soda, S. Takagi, in SID2004 Digest of technical papers, v.XXXY, book 1, 674-677, (2004), describe a colour in-plane EPD using an anisotropic scattering layer. The device provides an RGB display which uses a colour filter in the background and works in transmittance mode with a backlight. The display uses in-plane switching. One pulse cause migration of pigments which collect on spacer walls and light is transmitted through a relevant pixel with a colour filter. An opposite pulse provides opposite migration of the pigment particles, which cover an area under the pixel and block light transmission. The colour filters reduce brightness, which makes the display less effective for reflective mode.
Scattering and PDLC systems use dyes for colouring, for example as described by Akira Masutani, Tony Roberts, Bettina Shuller, Akio Yasuda, Akira Sakaigawa, Graham Cross, David Bloor in “A novel polarizer free dye-doped polymer dispersed liquid crystal for reflective TFT displays” Journal of the SID, v. 12, No 3, 301-308(2004). These systems also are monochrome (bicolour) and for multicolour switching a colour filter mask is needed.